The present invention relates to packet switched radio telephone services and is applicable in particular, though not necessarily, to the General Packet Radio Service (GPRS). Moreover, the invention concerns idle-mode cell re-selection measurements in GPRS.
Current digital cellular telephone systems such as GSM (Global System for Mobile communications) were designed with an emphasis on voice communications. Data is normally transmitted between a mobile station (MS) and a base station subsystem (BSS) over the air interface using the so called xe2x80x9ccircuit switchedxe2x80x9d transmission mode in which a series of regularly spaced time slots on one frequency band are reserved for the duration of the call. For voice communications, where the stream of information to be transmitted is relatively continuous, the circuit switched transmission mode is reasonably efficient. However, during data calls, e.g., internet access or facsimile transmission, the data stream is xe2x80x9cburstyxe2x80x9d and the long term reservation of time slots in the circuit switched mode represents an uneconomic use of the air interface.
Given that the demand for data services with digital cellular telephone systems is increasing rapidly, a new GSM based service known as the General Packet Radio Service (GPRS) is currently being standardised by the European Telecommunications Standards Institute (ETSI) and is defined in overall terms in recommendation GSM 03.60. GPRS provides for the dynamic allocation of transmission capacity for data transmission. That is to say that time slots on a frequency band (or bands) are allocated to a particular MS to BSS link only when there is data to be transmitted. The unnecessary reservation of time slots when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS will therefore uses a basic channel structure similar to hat defined for GSM. In GPRS, a given frequency band is divided in the time domain into multi-frames, each multi-frame consisting in turn of 52 TDMA (Time Division Multiple Access) frames. The length of a TDMA frame is 4.615 ms and each TDMA frame is in turn divided into eight consecutive slots of equal duration. This frame structure is illustrated in FIG. 1 and is relative to the transmission and reception time at the BSS.
In the conventional circuit switched transmission mode, when a call is initiated, two physical channels are defined for that call at the BSS by reserving two respective time slots, separated by two intervening slots, in each of a succession of TDMA frames. One of these channels provides a downlink channel for carrying user data from the BSS to the MS whilst the other provides the uplink channel for carrying user data from the MS to the BSS.
With the introduction of GPRS (the general architecture of a GSM/GPRS network is illustrated in FIG. 2) the fixed relationship between time slots allocated for uplink and downlink channels no longer applies. Time slots may be dynamically assigned to the uplink channel and the downlink channel for a given MS depending upon demand and capacity and MS multi-slot class. So, for example, in any given TDMA frame one time slot may be allocated to the downlink channel with two slots being allocated to the uplink channel. Also, there is no fixed time relationship between the uplink and the downlink allocated slots. Slot allocation is notified to the MS during a channel set-up stage.
There is illustrated in FIG. 3 a cell 1 of a cellular mobile telephone network. A mobile station (or telephone) 2 located within the cell 1 communicates with a base station (BS) 3 of the cell. As has already been described above, when a call or data connection is made from the mobile station 2 (also referred to as MS) to the BS 3 or vice versa, a downlink xe2x80x98channelxe2x80x99 and an uplink xe2x80x98channelxe2x80x99 are reserved to enable bi-directional communication to take place. The MS 2 is provided with a radio frequency part RF, with reception branch RX and transmission branch TX for performing transmission and reception functions. Transmission and reception is controlled by the MCU (Master Control Unit), which controls e.g. timing of performing the different known operations of an MS. Also the MS monitors the radio conditions in the serving cell and in neighbouring cells.
When a MS first connects to a GPRS cellular network, the MS synchronises itself to the BSS using information carried by a synchronisation channel (SCH) transmitted by the BSS to all listening MSs. Also when a MS first connects to a GPRS cellular network, the MS looks for a cell suitable cell by checking cells in descending order of received signal strength. If a suitable cell is found, the MS camps on it and performs any registration necessary. Cells can have two levels of priority, suitable cells which are of low priority are only camped on if there are no other suitable cells of normal priority. This is called xe2x80x9ccell selectionxe2x80x9d. When camped on a cell the MS regularly looks to see if there is a better cell in terms of a cell re-selection criterion, and if there is, the better cell is selected. Also if one of the other criteria changes, (e.g., the current serving cell becomes barred), or there is a downlink signalling failure, a new cell is selected. This is called xe2x80x9ccell re-selectionxe2x80x9d.
Procedures for cell re-selection are specified in GSM specification 05.08. Cellxe2x80x94re-selection measurements are performed when the MS is in idle mode, i.e. switched on but not having a dedicated channel allocated, e.g. not making or receiving a call, or when in group receive mode, that is, receiving a group call or broadcast call but not having a dedicated connection. Basically idle-mode can also be defined as any other period, except when the MS is in transfer mode. In packet idle mode the MS has to make measurements (signal strength mesurements, power measurements) for cell re-selection in order to ensure that a mobile is camped on a cell with which it can reliably communicate on both the radio uplink and downlink.
It is currently specified for GPRS idle mode cell re-selection measurements that a mobile station shall take one measurement sample on each carrier on the BA list (BCCH Allocation specifying the BCCH channels the MS shall listen to) and one sample on the own BCCH carrier for each paging block reception. The maximum number of measurements per second is 200.
It is recognised that the requirement for GPRS has to be higher than for circuit switched operation because of the nature of operation. The current cell re-selection measurement requirement, however, does not take into account the different DRX periods that MS may have and thus the optimum performance in terms of measurements per second with respect to average power consumption is not achieved. The MS receives paging blocks (PB) from the BS which the MS listens to to find out whether there are transmissions coming to it from the BS. The period between two consecutive PBs is called a DRX period (discontinuous reception). During the DRX period the MS may power itself down as it is not expecting paging messages from the network. The DRX mode is permitted at all other times in idle mode except while performing the cell selection algorithm. The length of the DRX period may vary from time to time, and the MS receives a parameter from the BS on basis of which the MS can calculate when it is to expect PBs, and as the MS is synchronised with the BS it knows exactly when the PBs are coming.
It is possible that in packet idle mode a MS receives several paging blocks (PB) during a multiframe, the requirement of measurements per multiframe can be several times the number of monitored carriers. In practice this means that the upper bound of the current GPRS solution, namely 200 measurements/s, will be reached easily. When the DRX period is very short power consumption increases unnecessarily because of xe2x80x9ctoo manyxe2x80x9d idle-mode cell re-selection measurements.
An object of the present invention is to disclose a method of improving operation in view of cell re-selection measurements, preferably in idle-mode cell re-selection measurements in GPRS.
In GSM circuit switched operation the number of measurements is dependent of the number of monitored carriers (BGCHs). In practice this means that in relation to the length of the DRX period the number of measurements is always substantially constant, namely seven measurements. Also in the currently existing proposal for GPRS the number of measurements is fixed in relation to the DRX period, whereby with a short DRX period the number is unnecessarily high increasing current consumption, and with a very long DRX period the number is too low whereby an accurate measurement result is not received often enough. According to the present invention it has been recognised that prior solutions for making cell re-selection measurements fail to take the length of the DRX period into account. Accordingly the present invention proposes to have the number of cell re-selection measurements depend upon the length of the DRX period, i.e. on how often PBs are sent to the MS. The number can vary either based on an equation having linear or non-linear relationship, or alternatively a fixed number of measurements can be specified in such way that for a shorter DRX period the number of measurements is lower and for a longer DRX period the number of measurements is higher. By this proportional relationship is meant that there is a substantial variation in the number of measurements upon a substantial change in the length of the DRX period. This is a clear difference to prior art, in which the number of measurements is substantially constant on different DRX period lengths.
According to a first aspect of the invention there is provided a method of controlling cell idle-mode re-selection measurements in a mobile station in a mobile communication system comprising base stations where a base station defines a cell, and a mobile station for communicating with at least one base station, in which method when the mobile station is in idle mode,
the mobile station monitors paging messages coming from the base station informing the mobile station of possible transmissions to come, the paging messages arrive at certain intervals, the interval defining a discontinous reception period (DRX period), and
the mobile station performs cell re-selection measurements on certain channels, characterized in that
the number of cell re-selection measurements is proportional to the length of the DRX period, whereby during a longer DRX period a higher number of cell re-selection measurements is performed than during a shorter DRX period.
According to a second aspect of the invention there is provided a mobile station for a mobile communication system comprising base stations where a base station defines a cell, and the mobile station comprises
communication means for communicating with at least one base station,
control means for controlling the communication of the mobile station,
reception means as part of the communication means for receiving paging messages, when the mobile station is in idle mode, from the base station informing the mobile station of possible transmissions to come, the paging messages have been arranged to arrive at certain intervals known to the control means, the interval defining a discontinous reception period (DRX period), and
measurement means for performing cell re-selection measurements under control of the control means on certain channels when the mobile station is in idle mode, characterized in that
the control means has been adapted to control the number of cell re-selection measurements to being proportional to the length of the DRX period, whereby the control means is adapted to control a higher number of cell re-selection measurements to be performed during a longer DRX period than during a shorter DRX period.
By the definition monitoring paging messages (Paging Blocks, PBs) is meant reception of PBs, or at least a trial to receive a PB. Thereby a DRX period is defined by two consecutive PBs which are intended for the particular MS, not dependent upon whether the actual reception of each such PB is successful or not.